Friday will offer the opportunity in some regions of the globe to observe a rare trifecta of celestial events: the March equinox will be accompanied by a total solar eclipse and a “supermoon” in the night sky.
The eclipse will be visible in its totality only in some parts of the Asian Arctic and northern Europe. Most of the rest of Europe and parts of Africa will experience a partial solar eclipse on Friday.
Total solar eclipses are not uncommon, as they occur about once every 18 months, but they occur only when the moon moves in front of our star and is aligned with the Earth during its orbit about our planet.
The moon’s orbit is inclined at about 5 degrees to the Earth’s orbit around the sun. That means the moon most often passes beneath or above the plane of the sun during its orbit of Earth.
Syzygy, as the alignment of the moon with both Earth and the sun is known, can only take place during a new moon.
While the sun is hundreds of times larger than the moon, the moon is hundreds of times closer to the Earth than the sun. The shadow cast by the moon during the total eclipse, called an umbra, will block out nearly all of the sun’s light and could persist on Friday for as long as about seven minutes.
During a partial solar eclipse a part of the moon’s shadow called the penumbra is cast over a portion of Earth.
According to a website maintained by NASA’s Goddard Space Flight Center, residents of North America will not have another opportunity to see a total eclipse of the sun until Aug. 21, 2017.
The occurrence of a total solar eclipse on the same day as the March equinox is unusual. One recent post at EarthSky.org reported that this confluence of the two events will happen this century only this year and in 2034, 2053, and 2072.
The “supermoon” is really just the appearance of a larger moon. In reality, the phenomenon occurs when Earth’s satellite reaches the perigee of its orbit. When the “supermoon” occurs, the moon is about 52,000 kilometers closer to Earth than it is at the apogee of its revolution.
Because the moon is that much closer at its perigee, its angular diameter appears from Earth to be about six one-hundredths of a degrees greater than it does at apogee.
The March equinox marks the beginning of spring in the northern hemisphere and of autumn in the southern hemisphere. On that day both hemispheres receive an equivalent amount of sunlight.
Any person who plans to look at the sun during an eclipse should use a pinhole camera to do so. An alternative is shade 14 arc welder’s glasses.
It is not safe to look at an eclipse through ordinary sunglasses, film negatives, or polarizing camera filters.
This NASA website is a useful resource for information about safe viewing of the sun during an eclipse.
NASA is set to launch this evening a probe to study Earth’s magnetosphere. The Magnetospheric Multiscale Mission will launch at 8:44 pm MDT.
Ganymede, the largest moon of Jupiter, is a water world.
Researchers using the Hubble Space Telescope have confirmed an ocean beneath Ganymede’s icy crust that are likely ten times deeper than Earth’s oceans.
The moon may have more liquid water beneath its surface than there is on Earth.
To reach this conclusion, scientists measured fluctuations in the moon’s aurorae. Aurorae, which are electrified ribbons of heated gas, are generated when a planetary body’s liquid metal core produces a magnetic field.
On Ganymede, the aurorae around its north and south poles move, influenced by fluctuations in Jupiter’s huge magnetic field.
Scientists used computer modeling to verify that the only explanation for the extent of the “rocking” of Ganymede’s aurorae is the presence of a saline ocean, which would cause Ganymede’s magnetic field to offset to some extent Jupiter’s magnetic impact.
Several measurements indicated that the aurorae moved by about two degrees, but without the presence of an ocean Jupiter’s influence would have caused movement of about six degrees.
“When there is an electrically conductive ocean present, this counteracts Jupiter’s influence,” Joachim Saur, a professor geophysics at Germany’s University of Cologne and the lead researcher, said.
Saur explained that four independent measurements with HST verified the two-degree movement of Ganymede’s aurorae.
Scientists had speculated for several decades that Ganymede could be an oceanic moon. The Galileo probe, which flew by the moon several times during its 1995-2003 mission, provided further grist for that supposition when it measured the moon’s magnetic field. But those flybys did not last long enough for scientists to detect the movement of Ganymede’s aurorae.
“The flybys lasted only 20 minutes each,” Saur said. “In the new Hubble observations we have seven hours of data, so we do not have the ambiguity anymore.”
Ganymede’s sea may also be impacting the surface of the moon. Mapping of the satellite by the U.S. Geological Survey reveals areas of the surface that are smoother and less cratered than other areas, which could indicate the presence of tectonic forces that permit movement of sea water from beneath the icy crust.
“These lighter shaded regions are believed to be formed by flooding on the surface by water coming to the surface by faults or even cryovolcanoes,” Jim Green, NASA’s director of planetary science, said.
Ganymede is Jupiter’s largest moon, with more mass than Mercury. It joins fellow Galilean moon Europa and Saturn’s moon Enceladus on the list of moons known to have water oceans.
Another of Jupiter’s four largest moons, Callisto, is also thought likely to have liquid water.
“Every observation that we make, every mission that we send to various places in the solar system, is taking us one step closer to finding that truly habitable environment, a water-rich environment in our solar system,” Heidi Hammel, executive vice president of the Association of Universities for Research in Astronomy, said. “Everywhere we look there’s water.”